I have a seven-year-old cousin (girl) named Sullivan. She goes by her full name and by "Sully" on occasion. While it's not my favorite name, it's better than my other baby cousins' names (Kayla and Braxton).

In short, if you like it, use it! Names only have gender associations because they were dubbed so at one point. I play pretty loose and fast with the naming rules...which aren't really rules at all, if you get down to it.